In conventional oil drilling operations, it is known for bore holes to extend for many kilometers and for the drill bit to be steered, so that the direction of the bore hole may change along its length. The length of such bore holes and the deviation of the bore hole results in enormous loads on the drill string as it rotates. To reduce drag in the bore hole, the drill string is of a considerably smaller diameter than the bore hole and the bore hole is only contacted along its length by the drill bit and by various drill string tools, such as drill string stabilisers which stabilise the drill string relative to the bore hole, and cutting bed impellers which lift cuttings from the lower side of the bore hole to the upper side of the bore hole where they can be raised to the surface under the action of a continuous flow of drilling mud. In certain circumstances, such as where successive down hole tools are working in rock strata of different hardness, the down hole tool working in the weaker strata may become unstable and may gouge the sides of the bore hole, increasing the amount of material which must be carried to the surface and reducing the local stability of the drill string. The present invention has been created to alleviate this problem.